The occurrence of haze in lubricating base oils which are stored for some time is a generally recognised problem. Both solvent dewaxed and catalytically dewaxed lubricating base oils are known to produce haze after having been stored for some time. The haze is thought to be caused by traces of high melting waxy molecules, particularly linear and slightly branched paraffins, which are still left in the base oil after the dewaxing treatment. It will be appreciated that the occurrence of haze makes the lubricating base oils less commercially attractive.
Several methods have been proposed in the art to deal with the problem of haze.
For instance, in U.S. Pat. No. 4,269,695 a catalytic dehazing process is disclosed, wherein contaminated dewaxed lube base stock oil is contacted in the presence of hydrogen with a catalyst comprising a hydrogenation component, suitably nickel, and an aluminosilicate zeolite having a silica/alumina molar ratio of at least 12. Suitably, the zeolite should also have a framework density of more than 1.6 g/ml and a constraint index of from 1 to 12. An inorganic porous matrix material may also be present and the preferred material for this purpose is alumina.
In U.S. Pat. No. 4,428,819 a catalytic dehazing process is disclosed, wherein dehazing is accomplished by isomerisation of the waxy molecules which are held responsible for the haze formation. To this end the lubricating base oil feed is contacted in the presence of hydrogen with a catalyst comprising a hydrogenation component, suitably a noble metal-based one, and a crystalline zeolite having high structural silica to alumina molar ratios, i.e. 50:1 or higher. The background of this patent is that the isomerisation reaction only requires a relatively small degree of acidic functionality. Higher degrees of acidic functionality would only favor the undesired cracking reactions. Crystalline zeolites in a highly siliceous form should, accordingly, be used. In addition to the hydrogenation component and the crystalline zeolite a porous inorganic refractory oxide binder material, preferably alumina, may be present as well. Hydroisomerisation conditions involve a temperature of from 200 to 450.degree. C. and a pressure of from 4 to 250 bar. The process is particularly suitable for dehazing catalytically dewaxed base oils having a boiling point above 345.degree. C.
In U.S. Pat. No. 4,867,862 a process is disclosed wherein a multilayered catalyst system is used for hydrodehazing and hydrofinishing a hydrocracked, solvent dewaxed lubricating base oil. The hydrodehazing catalyst should have a high selectivity for normal paraffins as related to branched paraffins and may be selected from the catalysts conventionally applied in catalytic dewaxing processes. The use of a catalyst comprising a silicoaluminophosphate (SAPO) is preferred. The hydrodehazing catalysts may suitably comprise a porous inorganic refractory oxide--alumina being preferred--and may or may not contain a hydrogenation component. The liquid hourly space velocity in the hydrodehazing step should be high, i.e. greater than 4 hr.sup.-1, while temperature and pressure are about 290 to 345.degree. C. and greater than about 35 bar, respectively.
Although the prior art processes described above may perform satisfactorily in many respects, there is still room for improvement. The present invention, accordingly, aims to provide a process for catalytically dehazing those lubricating base oils suffering from a haze problem when stored for some time in an effective way. The present invention also aims to provide a catalytic dehazing process which can be carried out at relatively mild conditions. Furthermore, the present invention aims to provide a catalytic dehazing process which can be readily integrated with existing dewaxing operations, such as catalytic dewaxing and solvent dewaxing operations.